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1.
CBD方法对天然样品磁性矿物影响   总被引：2，自引：1，他引：1       下载免费PDF全文
本研究选择川西高原、天山和西伯利亚Kurtak剖面的黄土古土壤样品及亚热带非风成样品进行CBD处理,系统测量并对比处理前后的磁学参数,包括低频磁化率、频率磁化率、非磁滞剩磁、饱和等温剩磁、剩磁矫顽力和热磁曲线(J-T曲线),分析处理前后磁性矿物种类、含量和磁畴的变化.结果表明,CBD方法对于磁性矿物的溶解并无明显的选择性,在温度与反应时间一定的条件下,磁性矿物的溶出量主要受控于其粒径分布.CBD方法可以非常有效地去除具有更大比表面积的细粒(< 1 μm)磁性矿物,同时溶解粗粒(>1 μm)磁性矿物外缘,使其粒径变细.CBD处理后磁化率变化存在多种可能,对于成土作用较强的古土壤,CBD方法可以较为准确地提取成土成因的磁性信息;而干旱和过度湿润条件下的风积黄土,不宜使用CBD方法区分原生与次生磁性矿物.  相似文献   

2.
风成黄土是陆地上分布最广泛的沉积物之一,记载了各种古气候演化信息.目前巴基斯坦的黄土研究甚少,磁化率与气候对应的变化机制研究尚未开展.本文对位于巴基斯坦印度河平原Bahawalpur地区新发现的黄土-古土壤剖面进行系统的岩石磁学研究,结合粒度和漫反射光谱(DRS)数据,讨论巴基斯坦黄土的磁化率变化机制.实验结果显示:Bahawalpur(BH)剖面黄土层主要的载磁矿物为磁铁矿,同时含有少量磁赤铁矿和针铁矿,磁性颗粒以原生的MD和PSD颗粒为主.相对于黄土层,古土壤层则是以针铁矿为主,含有顺磁性矿物和少量磁铁矿.BH剖面磁化率与成土作用关系和中国黄土高原典型剖面相反,磁化率的变化可能存在一个阈值12.8×10-8 m3·kg-1,在阈值之上,强磁性矿物(磁铁矿、磁赤铁矿)占主导;阈值之下,以弱磁性矿物(主要是针铁矿)为主,这种磁性矿物的转变可能导致磁化率降低.本文可为今后利用磁化率解读该地区地层蕴含的古气候信息提供新线索.  相似文献   

3.
The magnetic properties of loess, lake, and ocean sediments are often used as indicators for paleoclimatic/paleoenvironmental changes. Thermomagnetic analysis is a conventional approach for identifying magnetic phases and thermal alteration of samples. Magnetic concentration parameters are often enhanced after thermal treatment. In this study, the role of clay minerals in magnetic enhancement at elevated temperatures is systematically investigated. The results indicate that the clay minerals (saponite, Ca-montmorillonite, kaolinite, and chlorite) are dominated by paramagnetic behaviour and that the magnetic properties remain relatively stable after heating to 700°C in argon. In contrast, mixtures of hematite and chlorite have a high degree of magnetic enhancement after heating in argon, which indicates that clay minerals play important role in magnetic enhancement. These results improve our understanding of the processes involved in complicated mineral transformations, which is important for retrieving paleoclimatic/paleoenvironemntal signals from magnetic proxies.  相似文献   

4.
对甘肃西山坪遗址剖面样品进行磁化率、频率磁化率、热磁分析、非磁滞剩磁和等温剩磁等一系列岩石磁学实验测试,分析环境磁学参数特征变化,试图探讨西山坪遗址研究意义,为今后深入研究该沉积序列蕴含的古环境记录提供了岩石磁学基础.结果表明：西山坪遗址剖面沉积物中磁性矿物主要以低矫顽力的磁铁矿和磁赤铁矿为主,高矫顽力磁性矿物赤铁矿含量极其有限,越向剖面表层,强磁性矿物含量逐渐增加.磁性矿物颗粒表现出黄土的典型特征,以单畴、准单畴为主,含有一定量的超顺磁颗粒,颗粒度越向剖面表层越细.从物源上看,遗址沉积物主要来自近源的黄土风尘堆积,主要是风成堆积,并混合大量人类活动信息,给今后综合研究岩石磁学与人类活动特征的关系有一定借鉴意义.  相似文献   

5.
The dominant magnetic minerals and carriers of magnetic signals within the Chinese Loess Plateau are magnetite, maghemite, hematite, and goethite. In this study, we investigated the provenance and evolution of magnetic minerals during loess pedogenesis, using X-ray diffraction (XRD) and optical and electron microscopy, including field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). Our results reveal that single- and multiphase mineral assemblages among magnetic minerals in the loess-paleosol sequence have been formed. Partial oxidation of coarse eolian magnetite has occurred in the desert source area and the oxidation degree is enhanced after deposition of the dust upon the Chinese Loess Plateau. This mode of origin resulted in a microtexture consisting of an inner magnetite core surrounded by a hematite rim, and strongly affected the magnetic characteristics of the loess. Goethite coexists with hematite in the loess and paleosol, and nanometer-scale hematite is formed upon goethite rims via dehydration. Our study provides direct mineralogical evidence of the magnetic record and paleoclimatic implications of the loess-paleosol sequence of the Chinese Loess Plateau. Supported by National Natural Science Foundation of China (Grant Nos. 40772032 and 40573054) and National Basic Research Program (Grant No. 2007CB815603)  相似文献   

6.
以黄土高原西北缘的靖远和古浪剖面(包含黄土层L1上部和占土壤层SO)作为研究对象,选取代表性样品进行磁化率、频率磁化率、热磁曲线、等温剩磁获得曲线和磁滞回线等测定.结果表明,靖远和古浪L1黄土和SO古土壤具有相似的岩石磁学特征.磁性矿物含量相对较低,载磁矿物均以磁铁矿为主,同时含有磁赤铁矿和赤铁矿,且SO占土壤中的磁赤...  相似文献   

7.
Aeolian loess deposits contain abundant information about the evolution of the paleoenvironment.For example,paleoclimate changes recorded in Chinese loess area obtained significant achievement in the past few decades.Compared to Chinese loess,research on Indian loess is lacking.Currently,most studies focus on the Kashmir area located in the southern Himalayas,and studies on other areas are rare.However,field observations demonstrate that the sediments around the New Delhi-Agra-Jaipur Plain are similar to Chinese loess-paleosol sequences.For example,the boundary between two strata is transitional and without horizontal bedding.Moreover,obvious pedogenic horizons developed among sediment sequences,probably indicating unrecognised aeolian deposits in the Indo-Gangetic Plain(IGP).To confirm this,pilot samples were obtained from the IGP and detailed indoor measurements conducted.The results indicate that the distribution patterns of particle size and rare earth elements(REE) of the pilot samples are similar to Chinese loess.Furthermore,the scanning electronic microscopy(SEM) images of pilot samples show obvious conchoidal fractures,dash-shaped concavities,and abundant small pits that usually form through mechanical impact.These are typical characteristics of aeolian particles.In addition,environmental and rock magnetic measurements indicate that the dominant magnetic minerals in the pilot samples are magnetite and maghemite,and that they likely contain small amounts of hematite.Furthermore,conventional magnetic parameters are comparable with Chinese loess.Based on this,aeolian loess deposits are widely distributed in the IGP,which may have promoted the development of Indian farming and contributed towards the prosperity of ancient Indian civilisation.This study also provides a new and valuable record for the research on paleoclimate changes in the study area in the future.  相似文献   

8.
Morphological characteristics and microstructures of magnetic minerals extracted from Chinese loess and paleosols were investigated using powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Our results indicate that maghemite in loess–paleosol sequences was transformed from magnetite through oxidation of magnetite. Maghemite transformed from eolian magnetite during chemical weathering has low-angle grain boundaries among maghemite nano-crystals. Some nano-crystalline maghemites with nanoporous texture resulted from microbe-induced precipitation of magnetite or transformation of poorly crystalline ferric Fe (oxy)hydroxides in presence of Fe-reducing bacteria. Aggregates of euhedral maghemite nano-crystals were transformed from magnetite magnetosomes. Both microbe-induced nanoporous magnetite and microbe-produced magnetite magnetosomes are directly related to microbial activities and pedogenesis of the paleosols. It is proposed that the formation of nano-crystalline maghemite with superparamagnetic property in paleosol results in the enhancement of magnetic susceptibility, although the total amount (weight percent) of magnetic minerals in both paleosol and loess units is similar. Our results also show that nano-crystalline and nanoporous magnetite grains prefer to transform into maghemite in semi-arid soil environments instead of hematite, although hematite is a thermodynamically stable phase. This result also indicates that a decrease in crystal size will increase stability of maghemite. It is also inferred that surface energy of maghemite is lower than that of hematite.  相似文献   

9.
黄土岩石磁学参数是古气候研究中的重要指标,其中磁化率应用最为广泛,并在黄土高原地区取得重大进展,其受控于成壤作用的变化机制也被普遍接受.然而在黄土高原外缘的新疆地区,磁化率的变化机制仍不明确,导致磁化率的古气候意义在该区存在较大争议.本文选取塔里木盆地南缘具有精确年代控制的典型黄土剖面（羊场剖面）开展岩石磁学和高分辨率磁化率研究,利用交叉小波分析方法并结合剖面粒度、矿物及元素特征对该地区磁化率变化机制进行初步探讨.结果显示,羊场剖面的岩石磁学性质主要由粗颗粒软磁性矿物所控制,同时也表现出一定的顺磁性特征.根据载磁矿物和磁化率变化特征可将剖面进一步划分为两个阶段：阶段Ⅰ（8.5~2.5 ka）,载磁矿物以亚铁磁性的磁铁矿为主,磁化率值整体较高;阶段Ⅱ（2.5~0.2 ka）,亚铁磁性矿物依然占据主导地位,但硬磁性矿物和以黄铁矿为代表的顺磁性矿物相对增多,磁化率值显著降低.相关性研究和交叉小波分析表明：阶段Ⅰ磁化率与粗颗粒组分的变化具有一致性,符合"风速论"模式;阶段Ⅱ磁化率不仅与粗颗粒组分具有明显的正相关关系,而且与指示成壤作用强度的频率磁化率百分含量呈现出显著的负相关关系,暗示了阶段Ⅱ的磁化率变化可能受到"风速论"和"还原性成壤"模式的共同影响.本文拓宽了对新疆地区黄土岩石磁学特征及其磁化率变化机制的深入理解,也为利用磁化率恢复新疆及中亚地区全新世以来的古气候变化历史提供了新的线索.  相似文献   

10.
The magnetic susceptibility (MS) of Chinese loess showing a general proportional relationship to pedogenic grade has been widely recognized and used for reconstruction of paleoclimate by Quaternary scientists. The in-situ pedogenic enhancement of ferrimagnetic content is normally believed to be the main reason for the increase of susceptibility in soil units. However, this pattern of high magnetic susceptibility in palaeosols, and low values in loess, are not replicated in some loess deposits. Siberian loess deposits display a completely opposite susceptibility behavior: high values in loess and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. Our magnetic study of Siberian samples in this paper suggests that there are notable differences in magnetic properties between Siberian loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult to explain fully through variation in wind strength alone, but implies that the low magnetic susceptibility values in the Siberian paleosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes. The Loess Plateau is a very arid area where potential evaporation is always higher than precipitation; pedogenesis occurs under dry oxidising conditions. The Siberian Kurtak region is located on the edge of the tundra where it is always wet and saturation during interglacials will lead to a reducing pedogenic environment. Ferrimagnetic minerals under this condition will be destroyed, resulting in lower magnetic susceptibility. Therefore, great care should be taken when using susceptibility values for paleoclimatic reconstruction.  相似文献   

11.
In most Chinese loess–paleosol sequences, high magnetic susceptibility values are found in the soil horizons, with low values in the loess layers. The susceptibility signal has been widely used as a proxy climatic indicator. However, both the causes and mechanisms which control susceptibility still remain controversial. Our recent studies challenge some earlier interpretations of the magnetic susceptibility signal in Chinese wind-blown sediments. First, the prevailing hypothesis of pedogenic origin cannot totally account for the susceptibility variations in many sections. Second, in some cases, the principal carrier of the magnetization seems not to be the ultrafine pedogenic minerals, rather coarse lithogenic magnetic minerals derived from local sources can also contribute significantly to the susceptibility signal. Finally, not all the Chinese soils have higher susceptibility values than that of loess or sand layers, opposite relationships do exist. It is concluded that the source and causes of susceptibility may be different from site to site or even at different times at a given site. Any single hypothesis cannot completely account for the susceptibility variations in the Chinese Loess Plateau and its surrounding regions. Besides the previous explanations of carbonate leaching, pedogenic processes and organic decomposition, the contribution of source materials to magnetic susceptibility must also be taken into account.  相似文献   

12.
Chinese loess–palaeosol sequences are well known for their records of monsoonal climatic variations. However, the modern processes of dust accumulation and soil formation remain poorly understood. A high‐resolution investigation on modern soils, including the measurement of magnetic susceptibility, particle‐size distribution, total Fe, total organic carbon, CaCO3 content, and optical stimulated luminescence (OSL) dating was carried out on the Zhouyuan loess tableland in the southern Loess Plateau. The results indicate that modern cinnamon soils (luvisols) have developed on contemporarily accumulated aeolian dust during the Holocene. The aeolian loess accumulated during the Younger Dryas was identi?ed in the top part of the Malan Loess that underlay the modern soil by OSL dating and proxy climatic data. It indicates that the Malan Loess accumulated during the last glaciation (marine isotope stages 2–4) does not serve as the parent material for the modern soils. Pedogenesis of the soils started with the increased precipitation and soil moisture that have occurred on the loess tableland since the early Holocene. Precipitation‐driven pedogenesis and organic activities are responsible for the leaching of CaCO3, decomposition of mineral dust and the production of clay and ferromagnetic minerals. Drier intervals have interrupted soil formation several times, and therefore pro?les with multiple soils have been developed at many sites on the loess tableland. At places where soil erosion was relatively strong, either a single soil or welded soils are preserved in the Holocene pro?les. This does not necessarily mean, however, that modern soils over the plateau have been developed without interruption under a constantly warmer, moister climate. This is signi?cant for understanding the surface processes and climatic variation during the formation of the numerous palaeosols over the Loess Plateau in the Quaternary. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

13.
极性转换期间地球磁场形态学研究   总被引：3，自引：1，他引：2  
本文对采自中国黄土高原西峰（35.7°N,107.6°E）和段家坡（34.2°N,109.2°E）两个剖面中黄土层L8和古土壤层S8的1281块定向古地磁样品做了详细的岩石磁学和古地磁学研究.证实了Matuyama-Brunhes（M-B）极性转换带位于L8的中下部.提出了下列观点:1.M-B极性转换过程与地球磁场方向变化相联系的持续时间为3600-4500a,而与地球磁场强度变化相联系的持续时间则为8000-9000a,即强度变化存在“超前和滞后” 效应;2.M-B转换场的形态是由三次快速倒转和一次不成功的倒转构成,或者说,转换场具有快速变换极性的振荡特征;3.M-B转换过程中地球磁场并不是以轴对称的非偶极子场为主,而是偶极子场至少与非偶极子场相当;4.中国黄土-古土壤沉积物所含磁性矿物的主要成分是磁铁矿,它是研究极性转换期间地球磁场详细结构的良好物质.  相似文献   

14.
三门峡盆地晚新生代沉积物磁性载体类型   总被引：3，自引：2，他引：3       下载免费PDF全文
对三门峡盆地晚新生代沉积岩样品进行岩石磁学研究,通过三轴饱和等温剩磁和剩磁矫顽力实验、交变退磁和热退磁实验及磁化率测定,发现黄土-古土壤、河湖相灰绿层和冲洪积层3种不同岩性的磁载体存在显著差异.即黄土-古土壤以磁铁矿为主,赤铁矿和磁赤铁矿很少;河湖相灰绿色沉积磁性矿物含量较低,主要为赤铁矿和磁铁矿,但磁铁矿较多,此外还有一些不稳定磁性矿物(如针铁矿、菱铁矿等);冲洪积物以磁铁矿和赤铁矿为主,磁铁矿相对较多.  相似文献   

15.
天山北麓黄土环境磁学特征及其古气候意义   总被引：1，自引：0，他引：1       下载免费PDF全文
新疆黄土-古土壤序列环境磁学参数的变化机理及其气候意义仍存在争议.本文选择天山北麓的中梁黄土剖面,系统开展了低温和常温下环境磁学参数的测试与研究,测量包括室温的磁化率与饱和磁化强度,以及磁化率与饱和剩余磁化强度的低温变化.结果发现,该剖面黄土和古土壤样品的磁性矿物主要由磁铁矿与磁赤铁矿组成,不含任何粒级成壤形成的超顺磁矿物颗粒,其磁化率信号主要记录了粉尘磁性矿物含量变化,较高的磁化率指示较强的风动力状况或者较近的风尘源区,新疆黄土的这种环境磁学"风尘输入模式"可用来重建干旱区的风动力强弱变化.  相似文献   

16.
Natural Magnetic Archives of Past Global Change   总被引：3，自引：0，他引：3  
Rock magnetic parameters of sediments reflect palaeoenvironmental and palaeoclimatic conditions during deposition in the marine and in the continental realm. The processes which lead to enhancement or degradation of the magnetic properties are well known – though not completely understood. They include variable terrigenous material influx, organic and inorganic in situ formation of ferromagnetic minerals and diagenetic destruction of these minerals. Cyclical changes in the magnetic mineral assemblages occurring at the orbital periodicities involved in standard Milankovitch theory have been observed in numerous sedimentary records, at least during the last seven million years. This confirms the relationship between rock magnetism and past global change. Quantitative interpretation of the magnetic signals, however, is still in its infancy and is hampered by the complex interplay of the signal formation processes.  相似文献   

17.
陕西交道全新世黄土-黑垆土磁化率的CBD研究   总被引：16，自引：7，他引：9  
根据黄士高原中部交道剖面全新世黄土－黑垆土剖面的非磁滞磁化率和ＣＢＤ提取前后的磁化率、频率磁化率以及热磁分析结果,分析了磁性矿物的种类和磁畴状态,讨论了古土壤的磁化率增强,指出ＣＢＤ技术强有力的支持古土壤磁化率增强的土壤成因观点．虽然ＣＢＤ能非常有效地溶解土壤成因的磁赤铁矿,但并不是完全选择性地溶解土壤成因的磁铁矿颗粒．此外,通过对样品ＣＢＤ处理前后磁化率数据的分析,从土壤化学角度为黄土的风成说提供了新证据。  相似文献   

18.
浅议中国黄土磁化率的物理意义   总被引：5，自引：0，他引：5  
中国的陆相黄土地层连续地记录了第四纪以来的古气候变化。基于中国黄土磁化率曲线与深海氧同位素记录具有可比性这一事实，磁化率作为一种气候替代性指标在黄土研究中得到广泛的应用。但是，磁化率的物理意义仍不清楚。本文作者对陕西榆林蔡家沟剖面末次间冰期以来的风成沉积作了研究，指出黄土、古土壤的磁化率差异不仅与前人提出的碳酸盐淋失、空隙度变化、“就地”新生成磁性矿物等作用有关，而且还可能与有机质有重要关系。有机  相似文献   

19.
The use of the sulphate mass balance (SMB) between precipitation and soil water as a supplementary method to estimate the diffuse recharge rate assumes that the sulphate in soil water originated entirely from atmospheric deposition; however, the origin of sulphate in soil and groundwater is often unclear, especially in loess aquifers. This study analysed the sulphur (δ34S-SO4) and oxygen (δ18O-SO4) isotopes of sulphate in precipitation, water-extractable soil water, and shallow groundwater samples and used these data along with hydrochemical data to determine the sources of sulphate in the thick unsaturated zone and groundwater of a loess aquifer. The results suggest that sulphate in groundwater mainly originated from old precipitation. When precipitation percolates through the unsaturated zone to recharge groundwater, sulphates were rarely dissolved due to the formation of CaCO3 film on the surface of sulphate minerals. The water-extractable sulphate in the deep unsaturated zone (>10 m) was mainly derived from the dissolution of evaporite minerals and there was no oxidation of sulphide minerals during the extraction of soil water by elutriating soil samples with deionized water. The water-extractable concentration of SO4 was not representative of the actual SO4 concentration in mobile soil water. Therefore, the recharge rate cannot be estimated by the SMB method using the water-extractable concentration of SO4 in the loess areas. This study is important for identifying sulphate sources and clarifying the proper method for estimating the recharge rate in loess aquifers.  相似文献   

20.
Summary We have studied 454 oriented samples from seven loess outcrops in the Czech Republic for comparison and correlation of the magnetic properties with those of the loess profiles in China and Central Asia. Three sections at Sedlec (Prague), Zeměchy and Dolní Věstonice cover the time span of the last glacial-interglacial cycle. Loess from the middle Pleistocene period including pedocomplexes V and VI was studied at Sedlešovice (Znojmo) and Karlštejn. Loess of uncertain — possibly lower to middle Pleistocene age — was sampled at Sedlec near Mikulov. At Červeny Kopec (Red Hill, Brno) we investigated the oldest loess near the Matuyama/Brunhes (M/B) boundary. The characteristic remanence (ChRM) determined after magnetic clearning is of normal polarity throughout all sections except at Červeny Kopec. There, two polarity changes from reversed to normal were found within two sections initially thought to represent two successive stratigraphic intervals. However, the marked similarities between lithological, susceptibility, declination and inclination profiles lead us to suggest that the two polarity changes are in fact one and the same. The two sections are therefore laterally equivalent and both have recorded the M/B field reversal. The susceptibility variations were used to mutually correlate the Czech sections and to construct a composite profile across the present day loess outcrops in the Czech Republic. They also serve as a paleoclimate proxy which can be compared and correlated with the paleoclimatic records observed in the Chinese and Central Asian loess. The origin of the low field susceptibility variations was studied by measuring the frequency dependence of susceptibility which revealed that the enhancement within the pedocomplexes is controlled by the amount of fine-grained ferromagnetic minerals present.  相似文献   

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